Recent microscopic studies of gingival biopsies from humans with advanced periodontal disease have shown the presence of many motile bacteria in the periodontal tissues (Saglie et al., 1982). It has been suggested that these bacteria originate from within the periodontal pocket and their movement into the gingival tissues may represent a key event in the progression of destructive periodontal disease. If this is so, then what causes them to move into the supporting tissues? It is the specific aim of this investigation to test the hypothesis that ecological factors present in subgingival plaque can drive motile bacteria by chemotaxis from the periodontal pocket into the surrounding tissues. To test this hypothesis, an in vitro chemotaxis assay will be modified in order to reproducibly quantitate the movement of radiolabeled Treponema sp. This assay system will then be used to study the effects of a large group of bacterial nutrients and bacterial metabolic products upon the movement of these spirochetes. The hypothesis predicts that localized environmental changes occur in developing subgingival plaque which result in the increased chemotactic activity of motile organisms. This increased activity may signal the onset of active periodontal disease. The results of this study will be used to expand our understanding of the progression of periodontal disease. It is hoped that the identification of specific chemotactic factors in the subgingival plaque will eventually lead to the development of a reliable biochemical assay for the determination of periodontal disease activity.